This invention relates generally to optical systems with reduced color shift performance and more particularly to transmissive screens for use in rear projection systems.
FIG. 1 illustrates a prior art rear projection system 10. Typical rear projection systems include an optical projection apparatus (e.g. projector) 12 and a screen 14.
There are many different technologies that may be utilized in the projector 12. Liquid crystal devices or LCD""s are one example used in optical systems. Optical systems with LCD based projectors can supply bright (e.g. 1000-2500 Lumen), high resolution performance.
Rear projection screens transmit an image projected onto the rear of the screen into a viewing space. A rear projection screen 14 may be a sheetlike optical device with a relatively thin viewing layer that is placed at an image surface of the projector 12. Examples of rear projection displays are disclosed in PCT WO 99/064927, PCT WO 99/13378 and EP 783 133 (the entire contents of each of which are herein incorporated by reference).
A rear projection optical system typically includes a fresnel lens and/or a lenticular lens or sheet. Examples of such systems are disclosed in U.S. Pat. Nos. 3,712,707; 3,872,032; 4,379,617; 4,418,986; 4,468,092 and 4,509,823.
Front projection systems are also known in the art. They comprise a projector designed to project an image on a surface (the wall of a conference room or a screen). Overhead projectors are an example of a front projection system. Rear projection screens provide several advantages over front projection screens. Generally, it is easier to achieve desirable contrast features with a rear projection screen system. With a rear projection screen, the presenter cannot cast a shadow on the image and the projection equipment may be hidden from view (which also helps mask any acoustical background noise emanating from electrical components). Front projection systems have less capacity to absorb ambient light than rear projection systems.
One known rear projection screen comprises a thin, light diffusing layer (frosted or translucent glass) constructed by etching, sandblasting or otherwise roughening a smooth glass surface. Since the translucent surface scatters light, the image is viewable from a range of viewing angles. Screens that are merely translucent tend to strongly reflect ambient light incident on the front, viewing side, resulting in fading or washout of the projected image. As a result, this rear projection screen is sensitive to ambient lighting conditions.
U.S. Pat. No. 2,378,252 discloses a rear projection screen comprising an array of closely packed glass beads associated with a transparent support and a light absorbing layer. The glass beads perform lens-like functions to collect light projected from the rear of the screen and focus it to relatively small spots near the area where the beads contact the support. The glass beads contact the transparent substrate and thereby exclude most of the light absorbing material at the contact area location between the glass beads and support. Ambient light incident on the front surface of the support is absorbed by the light absorbing layer. As a result, the front side of the screen appears dark, except for the light transmitted through the glass beads.
Rear projection screens with glass beads are also disclosed in U.S. Pat. Nos. 5,563,738 and 5,781,344. Additional beaded screens and methods of making such screens are disclosed in commonly assigned patent applications PCT WO 99/50710 and PCT WO 98/45753.
The present invention is particularly suitable for use in an optical system that includes a projected image source and a screen. The screen has a plurality (e.g. an array) of refractive elements (e g. glass beads), an optional light transmitting substrate, an optional light absorbing layer for controlling ambient light rejection; and a polarized light management layer for controlling the color shift of the optical system.
The illumination source provides light having a first polarization state associated with a first color and a second polarization state associated with a second color. The first polarization state is distinct from the second polarization state. For example, the first color may be completely linearly polarized in the horizontal plane, and the second color may be completely linearly polarized in the vertical plane. As used herein in the context of linearly polarized light, when it is said that a first color has a polarization state that is different than or distinct from the polarization state of a second color, it is meant that, with respect to the same plane of incidence, the relative amounts of p-polarized and s-polarized light in the first color are significantly different than the amounts of p-polarized and s-polarized light in the second color (i.e. more than ten percent). Preferably, at least: one of the first and second colors is a primary color.
The present invention also contemplates light that is polarized in a state other than linearly polarized light. For example, the first color may be elliptically polarized light with a major axis of electromagnetic vibration; and the second color may also be eliptically polarized light with a major axis of electromagnetic vibration. The major axes of the first and second colors may be offset. The present invention has particular advantages when such axes are orthogonal or perpendicular to each other.
LCD projectors are capable of providing an optical system with significant brightness (e.g., a brightness of at least 1000 Lumens ANSI). While bright, the light provided by a liquid crystal device (LCD) projector is linearly polarized. It was recognized that some LCD based projectors present the green, red and blue components of light in two distinct states of polarization. The polarization state of the green light was perpendicular to the polarization state of the red and blue light. For example, green was present in the horizontal state. Red and blue combined (purple) was present in the vertical state. It was recognized that, when such an LCD projector was used with a conventional glass-beaded, rear projection screen, a varying color shift or color shift gradient was visible to the viewer when the screen was viewed off-axis in either the horizontal or vertical direction.
In one embodiment of the present invention, the polarized light management layer may comprise a color compensation coating. The color compensation coating reduces the color shift viewable by a user of the optical system. Preferably, the color compensation coating is a xc2xc wave coating. Alternatively, the color compensation coating may have a non-uniform thickness. Also alternatively, the color compensation coating may have a portion whose thickness is less than a xc2xc wave thickness.
In another embodiment of the present invention, the polarized light management layer comprises a color compensating, diffuse coating. The coating preferably comprises a polymer with particles (e.g. beads) embedded therein. The refractive index difference between that of the polymer and that of the beads is preferably small.
A Color Shift Test is disclosed herein to evaluate the color shift of an optical system that utilizes preferred embodiments of the present invention. Preferably, an optical system with a glass beaded screen according to the present invention exhibits an off-axis color shift of about 0.010 or less at off-axis angles of sixty (60) or more degrees when measured according to the Color Shift Test. Also preferably, the optical system exhibits an off-axis color shift of less than about 0.005 or less at off-axis angles of forty-five (45) or more degrees when measured according to the Color Shift Test. Surprisingly, a 45-70% reduction (depending on the off-axis viewing angle) of color shift was visible to the viewer when a glass beaded rear projection screen incorporating the present invention was used in conjunction with an LCD based projector that presented the green, red and blue components of light in two distinct states of polarization.
In another aspect, the invention comprises a rear projection screen for use with an optical system as described above.